A variety of commercial processes rely on the use of fluid separation techniques in order to separate one or more desirable fluid components from a mixture. In particular, various such processes may involve the separation of liquid mixtures, the separation of vapors or gases from liquids, or the separation of intermingled gases.
For example, in the production of natural gas, it is typically necessary for the producer to strip carbon dioxide from natural gas in order to meet government regulatory requirements. It is also typically desirable in many chemical processes for hydrogen to be removed and recovered from gaseous process streams.
The use of membranes for fluid separations has achieved increased popularity over other known separation techniques. Such membrane separations are generally based on relative permeabilities of various components of the fluid mixture, resulting from a gradient of driving forces, such as pressure, partial pressure, concentration and temperature. Such selective permeation results in the separation of the fluid mixture into portions commonly referred to as “retentate”, e.g., generally composed of slowly permeable components, and “permeate”, e.g., generally composed of faster migrating components.
Gas separation membranes are commonly manufactured in one of two forms: flat sheet or hollow fiber. The flat sheets are typically combined into a spiral wound element, whereas the hollow fibers are commonly bundled together in a manner similar to a shell and tube heat exchanger.
In typical spiral wound arrangements, two flat sheets of membrane with a spacer in between are joined, e.g., glued along three of their sides to form an envelope, i.e., a “leaf”, that is open at one end. Many of these envelopes are separated by feed spacers and wrapped around a mandrel or otherwise wrapped around a permeate tube with the open ends of the envelopes facing the permeate tube. Feed gas enters along one side of the membrane and passes through the feed spacers separating the envelopes. As the gas travels between the envelopes, highly permeable compounds permeate or migrate into the envelope. These permeated compounds have only one available outlet: they must travel within the envelope to the permeate tube. The driving force for such transport is the pressure differential between the low permeate pressure and the high feed pressure. The permeated compounds enter the permeate tube, such as through holes passing through the tube. The permeated compounds then travel through the tube to join the permeated compounds from other of the tubes. Components of the feed gas that do not permeate or migrate into the envelopes leave the element through the side opposite the feed side.
In hollow fiber elements, very fine hollow fibers are wrapped around a central tube in a highly dense pattern. In such a wrapping pattern, both ends of the fiber end up at a permeate pot on one side of the element. Feed gas flows over and between the fibers, and some components of the feed gas permeate or migrate into the fibers. Such components then travel within the fiber until it reaches the permeate pot, where it mixes with permeated components from other of the fibers. The components collected in the permeate pot exit the element through a conduit or pipe. Components of the feed gas that do not permeate or migrate into the fibers eventually reach the central tube of the element, which is typically perforated similar to the permeate tube in the spiral wound element. In this case, however, the central tube is for residual or retentate collection, not permeate collection.
As will be appreciated, each element type has certain advantages. For example, spiral wound elements typically can handle higher pressures, are more resistant to fouling and have a long history in service in natural gas sweetening. In contrast, hollow fiber elements typically have higher packing density. Consequently, a hollow fiber-based installation is typically more advantageous for membranes with low permeability.
In either case, the membranes, once produced into elements, are typically formed into modules or cartridges, e.g., a tube containing a plurality of membrane separation elements. Modules can be used singly or, more commonly, interconnected in series or parallel arrangements or arrays. Typically, an installation may have from at least two up to several hundred modules in an array. Each module has an input (e.g., feed) stream, an output or residual stream that contains the substances which did not pass through the membrane separation element, and a permeate stream which contains the substances which pass through, e.g., permeate through, the membrane separation element.
Many such separation applications require rather high pressures. In many instances the pressures in such processes are in the range of about 35 kg/cm2 to about 210 kg/cm2 (about 500 psi to about 3000 psi). In dealing with such pressures, besides having sufficient wall thickness, it is necessary to have good pressure seals. The various process flow streams (e.g., feed, residual and permeate streams) must remain properly separated. Any intermingling of these streams decreases the efficiency of the process.
Current spiral wound membrane gas separation pressure vessel configurations are a product of the reverse osmosis industry, where such configurations have been the standard for many years. This vessel concept was designed for applications where filament wound plastic tubes could be easily incorporated, e.g., the internal diameter of the plastic tube could be easily manufactured to exact dimensions as the membrane web was wound over a mandrel.
The gas membrane industry has needed to modify the standard reverse osmosis vessel configuration to meet or satisfy their unique process requirements. For example, larger permeate pipes or conduits may be incorporated into some vessel configurations in order to better handle the larger possible permeate flows. Further, in order to meet the typical high pressure operation requirements associated with the gas processing industry and the tight roundness and diameter specification required for efficient packing of multiple modules, steel tubing has been employed with the internal diameter of the steel tubing honed to an appropriately high surface finish, e.g., 125 RMS or less.
In addition, an end draw configuration, such as used in reverse osmosis processing, has also typically been utilized in gas membrane separation applications. Such end draw configurations typically result in the permeate header extending out from the ends of the pressure tubes. Thus, such configurations generally result in a membrane skid being made longer than otherwise desired or in a reduction in length of the membrane tubes used therein, with a corresponding reduction in the capacity of the unit.
One of the difficulties in building such membrane skids is the need to ensure that the permeate header lines up perfectly with the flange connections at the end of the membrane pressure tube. Increasing the number of modules in an installation increases the number of flange connections that must properly aligned with a permeate header thereby increasing the difficulty of interconnecting individual modules.
In addition, a common problem associated with the use of spiral wound membranes is that each module containing the membranes is typically required to be machined to a close tolerance in order to assure good pressure seals. As a result, the cost for each module can be significantly increased.
In view of increased demand for product gases such as sweetened natural gas and purified gases such as hydrogen and carbon dioxide, the current market for gas separation membrane systems has moved toward larger installations. One approach to meet such increased demand is to incorporate membrane modules having an increased diameter to accommodate higher fluid flow rates. Alternatively, such larger installations may incorporate more membrane modules per skid to meet process specifications. However, the number of membrane modules loaded on an individual skid is controlled by the height and space limits at the installation site and the structural and weight limitations of individual skids and skid foundations.
Further, each of the membrane modules loaded on an individual skid requires some physical separation to accommodate installation of the individual membrane modules. Typically, membrane separation installations are constructed using a number of membrane separation modules which are stacked vertically to form a skid and create the required membrane area to process a fluid. This design requires a multitude of external connections to feed each individual membrane module and remove the processed fluid. As a result, packing of such large systems may present a problem because of the need to accommodate the input, output and permeate ports of each module.
Further such individual skids are constructed using structural steel to support each set of membrane modules. Such structural steel supports, however, add weight to the overall membrane system and increase the area required to install each individual skid. Consequently, such larger systems are heavier and more expensive to manufacture due to the quantity of materials needed to produce the structural steel supports, as well as, individual tubes for each module. Such larger systems are also more complex due to the increased number of connections between the membrane modules and common headers used to deliver and remove fluids from the skid.
Thus, there is a need and a demand for separation systems which incorporate an increased number of membrane cartridges or modules in a given area. In particular, there is a need and a demand for separation systems which incorporate multiple membrane cartridges into a single pressure vessel.
There is also a need and a demand for separation systems having simplified process fluid stream connections. Further, for example, there is a need and a demand for separation systems that permit feed stream delivery to, residual stream removal from, and permeate stream removal from a multitude of membrane cartridges at a reduced number of locations.
There is a further need and a demand for separation systems that are less expensive to produce.